A priority control system for a construction machine in accordance with the prior art as shown in FIG. 1 includes:
first and second variable displacement hydraulic pumps (hereinafter, referred to as “first and second hydraulic pumps”) 2 and 3 connected to an engine 1 and a pilot pump 4;
a boom control valve 7 configured to control the drive of a boom cylinder 6, a bucket control valve 9 configured to control the drive of a bucket cylinder 8, and a traveling control valve 11 configured to control the drive of a left traveling motor 10, wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path 5 of the first hydraulic pump 2 so as to be connected to each other through a parallel flow path 5a; 
a swing control valve 14 configured to control the drive of a swing motor 13, an arm control valve 16 configured to control the drive of an arm cylinder 15, and a traveling control valve 18 configured to control the drive of a right traveling motor 17, wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path 12 of the second hydraulic pump 3 so as to be connected to each other through a parallel flow path 12a; 
first and second pressure generation devices 19 and 20 configured to output a control signal corresponding to a manipulation amount, respectively;
a shuttle valve 23 configured to output a larger pilot signal pressure selected from pilot signal pressures applied to the swing control valve 14 so that the swing motor 13 can be swung in a left or right direction in response to the manipulation of the first pressure generation device (i.e., swing manipulation lever) 19; and
a priority control valve 21 installed in a flow path 29 between the parallel flow path 12a on the second hydraulic pump 3 side and an inlet port of the arm control valve 16, and configured to be shifted to a throttle state or a throttle release state by a pilot signal pressure outputted from the shuttle valve 23 when the swing motor 13 and the arm cylinder 15 are simultaneously manipulated.
When the first pressure generation device 19 is not manipulated and thus the pilot signal pressure is not applied to the swing control valve 14, the priority control valve 21 is maintained in the throttle release state by the elastic force of the valve spring 21a (see FIG. 1), so that the opening amount of the spool of the priority control valve 21 is switched maximally. On the other hand, when the pilot signal pressure from the shuttle valve 23 is applied to the priority control valve 21 through the flow path 22 due to the manipulation of the first pressure generation device 19, an internal spool of the priority control valve 21 is shifted to the top on the drawing sheet to cause the priority control valve to be shifted to the throttle state.
When the swing motor generating a high load and the arm cylinder generating a low load to perform an arm-in driving operation are simultaneously manipulated, a load occurring in the arm cylinder 15 is relatively higher than that occurring in the swing motor 13. Thus, a flow rate of the hydraulic fluid discharged from the second hydraulic pump 3 supplied to the arm cylinder 15 having a relatively low load is higher than a flow rate as supplied to the swing motor 15.
In order to prevent this, when the swing operation and the arm-in driving operation are simultaneously performed, the priority control valve 21 is shifted to the throttle state so that a flow rate of the hydraulic fluid supplied to the arm control valve 16 from the second hydraulic pump 3 is reduced and a flow rate of the hydraulic fluid supplied to the swing control valve 14 is increased as much as the reduced flow rate of the hydraulic fluid.
For this reason, when the swing motor 13 generating a high load and the arm cylinder generating a relatively low load to perform an arm-in driving operation are simultaneously manipulated, simultaneous manipulability can be maintained.
In the meantime, even when the swing motor 13 generating a high load and the arm cylinder generating a high load to perform an arm-out driving operation are simultaneously manipulated, the priority control valve 21 is maintained in the throttle state by the pilot signal pressure applied thereto due to the manipulation of the first pressure generation device 19. As a result, there is caused a problem in that the flow path of the priority control valve 21 connected to the arm control valve 16 is reduced, leading to a degradation of the operating speed of the arm cylinder 15 and an unnecessary pressure loss, thereby causing a hydraulic energy loss.